U.S. Pat. No. 5,085,171 to Aulick et al, assigned to the same assignee to which this application is assigned, is directed to a compliant doctor blade having a thin metal outer layer on a grit surface which faces the developer roller. This replaces prior rigid doctor blades which therefore could permit the toner layer of the developer roller to vary with surface variations in the doctor blade itself and the developer roller it comes in contact with. Such variations in the toner layer result in corresponding variations in the visible image made by the toner, both print and graphics. A compliant doctor blade ideally eliminates such variations.
That compliant doctor blade, although successfully used, has an effective life limited by the wearing away of the outer metal layer, as the metal is a necessary electrical path to charge the doctor blade where it contacts the developer roller.
The purpose of the doctor blade where it contacts a sector of the developer roller is three fold: 1) to help charge the toner, 2) to uniformly meter the correct amount of toner onto the developer roller prior to development, and 3) to repel toner of the opposite potential (termed wrong sign toner) so that it does not pass the doctor blade and become simply wasted toner. The surface roughness of the doctor blade is important to improve interaction with both the developer roller and the toner on the developer roller. The electric continuity to the contact sector on the developer roller, not exceeding a certain resistance, is important to permit the electrical functions of the doctor blade.
In addition to the wearing away of the metal layer of the compliant doctor blade of the foregoing patent, use of such a configuration in longer life applications permits an accumulation of toner at the entry area to the nip of the doctor blade with the developer roller due to compression on the corner of the foam which is in the shape of a wedge. When this wedge forms, it interferes with the ability of the doctor blade to meter the correct amount of toner, resulting in print quality problems on specific gray scale patterns (patterns of small images or dots separated but closely spaced to give the visual appearance of gray). Furthermore, once this wedge of toner appears, toner tends to begin fusing into the nip area of the doctor blade and the developer roller. This further alters the metering capabilities, resulting in rapid and severe degradation in print quality.